Why did UK and France not declare war on the USSR

This question have buggered me for a few years, and I never seem to get a satisfactory explanation. The UK and France made a guarantee to Poland in the event of an attack from a hostile neighbor. The 1st of September Germany attacked Poland. War declarations followed on the 3rd and 4th September, by these two Great Powers of Europe.

On the 17th, USSR rolls into Eastern Poland and seals the nations fate. But wait! Weren’t UK and France obliged to come to the defense of Poland whoever attacked, so why no declaration of war against the USSR?


Good question, the key is the Treaty of Versailles with garanteed the existence of Poland in accordance to the 14 points plan. The Germans were signees and the Poles basically functioned as the Eastern threat to Germany with France being on the West.

The Soviets did not sign the Versailles treaty but the British did help in the early 1920s Soviet-Polish war. When the Soviets invaded in 1939 it was not mandatory to invade although the Allies considered military action against the Soviets.

Oh and a heartfelt welcome to the forum, hope you’ll enjoy!


It was also evident just from Mein Kampf alone that Hitler intended to turn the East in “living space” so to speak. The British eventually came to recognize that war between Germany and Russia was inevitable and that Russia would be a valuable ally in that regard. Thus it was a “lesser of two evils” approach. History Matters on YouTube has an episode that answers this exact question. You should check him out, highly recommend.


I also think that Hitler was driven by, maybe with some kind of madness, a Roman Empire building attitude with a long time perspective - unfortunately, the historical timing of his life made ww2 possible


The Soviets hadn’t yet directly attacked the UK or France so they probably thought they could kick that can down the road & focus on Germany for the time being, though plans had been drawn up by the British to bomb the oil production at Baku. If you thought Spring 1940 went bad for the Allies in real life, just imagine what would have happened if Soviet forces were there too. Would the British have been able to escape Dunkirk in such a scenario? Remember, the UK also didn’t declare war on Japan until they attacked Hong Kong. In the end, the Western Allies kicked the can long enough for Hitler to invade the Soviets.


If you go on You Tube you’ll get a video covering this topic on the channel Knowledgia. It was the first video I watched about the topic and it gave me great insights into the hows and whys of that whole ordeal.

France and Poland signed a treaty of friendship in 1921 after the Polish-Soviet war which stated that if one or the other were the victim of an unprovoked attack, the other would help.
In May 1939 Poland and France signed a new treaty binding the militaries of the two nations, but this treaty only named Germany as a potential aggressor, thus France was technically not obligated to declare war on the Soviet Union after the Soviets invaded Poland.

The treaty signed between Poland and the United Kingdom however referred to “hostilities with a European power”.
Britain and Poland then also signed a treaty in August 1939 promising mutual military assistance if either were attacked by a European country. This means Britain was technically obligated to go to war with the USSR under the terms of the August 1939 treaty.

However, this was signed by the British hoping to discourage German expansionist ideals.
Thus, a secret clause referred to an attack by Germany specifically.
However if the aggressor was a different European national then Britain and Poland would only to consult together on measures to be taken in common.
Still, under the terms of the pact Britain was legally obligated to declare war on the Soviet Union.

When the Soviet Union invaded the Polish ambassador to London did request assistance.
Lord Halifax said the following: “As regards Soviet aggression we were free to take our own decision and to decide whether to declare war on the USSR or not.”

Still, the British used the secure clause of the 1939 pact to justify not going to war with the Soviet Union saying that they only guaranteed Poland’s protection in case of German aggression.

But fun fact, the Polish was not aware of this secret clause. The British did consider making it public to explain their lack of action to the Poles, but they decided not to. The British also said in October 1939 that the Poles should have understood that the pact only covered Poland in case of German aggression.

British ambassador to Moscow wrote in a secret telegram on 18 September 1939
“I do not myself see what advantage a war with the Soviet Union would be to us…our war aims are not incompatible with reasonable settlement (in Poland) on ethnographic and cultural lines.”

There was also the fact that neither Britain nor France were really prepared for war. They declared war on Germany but I mean neither of them really did much of anything, hence the Phoney War.
Seeing this the USSR thought that the Allies wouldn’t make another empty declaration of war as with Germany, so they thought it was safe to invade Poland without repercussions.

Plus Germany and the Soviet Union were historic rivals, so many people in Britain and France thought that maybe they’ll pick a fight with each other later on.
They also knew about the non-aggression pact between Germany and the Soviet Union, but they figured it was not a military alliance and if they declared war on the Soviet Union it might result in a German-Soviet alliance.
Should this happen Germany would have the natural resources from the Soviet Union as well as their own large industry.

Pre-war the focus was mainly on Germany and their expansion. Neither Britain nor France cared much about the Soviet Union. Britain and France weren’t ready for a war with even Germany, so they didn’t want to add the Soviet Union to the list of enemies.

So France wasn’t technically obligated to declare war on the Soviet Union, but Britain could’ve legally declared war. But in the end the British did not.
Britain and France wanted to stop Germany. They didn’t necessarily want to protect Poland specifically, just stop Germany. They had no designs on the Soviet Union.
Their treaties were to stop German aggression, not Soviet aggression. One can even argue they didn’t care that much about Polish independence as long as Germany stopped its expansion.


This is a really interesting question. My personal opinion is a little simpler and maybe uninformed. I think they didn’t do so for two reasons:

  1. Russia was not adjacent to France and thus did not immediately threaten it. This is after 9/1 of course.
  2. Taking on Russia, who was seen as a sort of friend, after already taking on Germany would be too much to take on. Russia may have used an excuse such as to protect ethnic Slavs in Poland and not as a land grab (which of course it was) as it was underway anyway they let it go.

Welcome to the forum. And yes, often it is a good point to look for logic and simplicity. Also I think the scary experience from WW1 where a string of Alliances brought every to war with each other must have been on their minds.


Sean McMeekin addresses this question in his book ‘Stalin’s War’. Stalin invaded Poland when is was clear the Western Allies were not really doing anything to attack Germany, and had reason to suspect the Allies were too weakminded to go to war against another country, while not really fighting against Germany. Stalin disguised the invasion as protecting Ukrainians and Belorussians in eastern Poland. Stalin exchanged territory in central Poland that was for Russia in the Molotov-Ribbentrop-pact for Lithuania and that made it seem as if Russia was only taking peripheral parts of Poland. Sean McMeekin: The Good War? Myths of World War II - CSPI Podcast #11 - YouTube


I thank you for the warm welcome. However, I have asked this question on multiple forums over the years and I never received a documentary evidence that the guarantee for Poland was just against German aggression. France and UK made a calculated choice, and at this point it was nothing about humanitarian reasons. They were just playing geopolitics. By all measures, Stalin was already a mass murderer in a category of his own, while the Nazis didn’t get rolling until Operation Barbarossa.

So, pardon me, but I don’t buy it.

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Apart from the megalomaniac interpretations of “Mein Kampf”, of which I think is way overblown. And I’ve read that extremely boring book a few times. But the issue of “the lesser of two evils” is incomprehensible to me. Stalin had already murdered 10s of millions by 1939, and the Western leaders knew it. The Nazis went into genocidal mode by late 1941.

So as not to be a hindsight warrior, they would at least play the two against each other.


Do you have any documentary evidence of what you are stating? I’ve heard this explanation many times, but never seen anyone who could back it up.


Thanks a bunch. That podcast didn’t just answer my question about these strange choices of the war, but it was worse than I thought. I have already ordered the book, and it would be a shame if the TImeGhost army didn’t make a piece on it.


So crazy, I just posted this as a question before I really looked through this forum. There’s really nothing fascinating in what I consider to be the “real reason” they only declared war on Germany. Answer, they couldn’t fight a war vs the USSR too, they weren’t even ready for a war vs Germany which was evident when France was overrun in one month. AFTER that event, their only hope was that the USSR (and the USA) would eventually side with the allies to fight Germany, quite possibly the only way they would win the war post 1940. Barbarosa forced the USSR into the alliance, and the rest is history.

Of course the moral implications of this are what we are all really thinking about, the “why?”. As christiaan put it quite well, they didn’t give a damn about the humanitarian reasons, or the lives of the Poles per se, only balance of power and the checks they could provide on any European aggression by being their “ally in the region”. It’s SIGNIFICANCE, to me, is that this type of nation building is still commonplace today.